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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 22 February 2011 by Bart Verheggen

The greatest source of uncertainty in understanding climate change is arguably due to the role of aerosols and clouds. This uncertainty offers fertile ground for contrarians to imply that future global warming will be much less than commonly thought. However, some (e.g. Lindzen) do so by claiming that aerosol forcing is overestimated, while others (e.g. the NIPCC) by claiming that aerosol forcing is underestimated. Even so, they still arrive at the same conclusion…

Let’s have a look at their respective arguments. Below is a figure showing the radiative forcing from greenhouse gases and from aerosols, as compared to pre-industrial times. The solid red curve gives the net forcing from these two factors. The wide range of possible values is primarily due to the uncertainty in aerosol forcing (blue dotted line).

The greenhouse gas forcing (dashed red curve) is relatively well known, but the aerosol forcing (dashed blue curve) is not. The resulting net anthropogenic forcing (red solid curve) is not well constrained. The height of the curve gives the relative probability of the associated value, i.e. the net climate forcing is probably between 1 and 2 W/m2, but could be anywhere between 0 and 3 W/m2. (From IPCC, 2007, Fig 2.20)

NIPCC’s argument

The NIPCC report (a skeptical document, edited by Craig Idso and Fred Singer, made to resemble the IPCC report) says:

“The IPCC dramatically underestimates the total cooling effect of aerosols.”

They hypothesize that natural emissions of aerosol precursors will increase in a warming climate, causing a negative feedback so as to dampen the warming. Examples of such gaseous aerosol precursors are dimethyl sulfide (DMS) emitted by plankton or iodocompounds created by marine algae. They use these putative negative feedbacks to claim that

“model-derived sensitivity is too large and feedbacks in the climate system reduce it to values that are an order of magnitude smaller.”

Rebuttal

These are intriguing processes (the “CLAW hypothesis” first got me interested in aerosols, when I assisted with DMS measurements on some remote Scottish islands), but their significance on a global scale is ambiguous and highly uncertain. As a review article about the DMS-climate link says:

“Determining the strength and even the direction, positive or negative, of the feedbacks in the CLAW hypothesis has proved one of the most challenging aspects of research into the role of the sulfur cycle on climate modification.”

The NIPCC report exaggerates the uncertainty in climate science, but seems to put a lot of faith in elusive and hardly quantified processes such as natural aerosol feedbacks coming to our rescue.

Lindzen’s argument

“The greenhouse forcing from man made greenhouse gases is already about 86% of what one expects from a doubling of CO2 (…) which implies that we should already have seen much more warming than we have seen thus far (…).”

“How then, can it be claimed that models are replicating the observed warming? Two matters are invoked.”

The “two matters” he refers to are aerosol cooling and thermal inertia from the oceans (a.k.a. “warming in the pipeline”). He then proceeds to argue that both of these factors are much smaller than generally thought, perhaps even zero. E.g. on aerosols, Lindzen writes:

“a recent paper by Ramanathan et al (2007) suggests that the warming effect of aerosols may dominate – implying that the sign of the aerosol effect is in question.”

By neglecting the importance of these two factors, Lindzen argues that the observed warming implies a small climate sensitivity.

Rebuttal

While it is true that aerosols can warm and cool the climate (by absorption and reflection of solar radiation, respectively, besides influencing cloud properties), most evidence suggests that globally, cooling is dominant. Whereas Ramanthan et al (2007) don’t quantify the net aerosol effect (in contrast to Lindzen’s implicit claim), Ramanathan and Carmichael (2008) do. They estimate both the warming ánd cooling effects to be stronger than most other estimates, but the net forcing (-1.4 W/m2) is right in line with (even a little stronger than) the IPCC estimate (see the above figure). Taking into account realistic estimates of aerosol forcing and ocean thermal inertia, the earth has warmed as much as expected, within the admittedly rather large uncertainties. Ironically, it is exactly because aerosol forcing is so uncertain and because the climate hasn’t equilibrated yet that the observed warming since pre-industrial times is only a very weak constraint on climate sensitivity. Lindzen seems very certain of something that most scientists would readily admit is very uncertain.

Conclusion

So we have the peculiar situation that both of these approaches try to claim that climate sensitivity is small, but the NIPCC approach is to claim that aerosol forcing is very large (thus providing a negative feedback to warming), whereas the Lindzen approach is to claim that aerosol forcing is very small (thus necessitating a small sensitivity to explain the observed warming so far). Of course they can’t both be right, and probably neither of them are. Looking back at the figure above, both approaches are based on assuming that aerosol forcing is at the edge of the probability spectrum (as if it were some fudge factor), whereas the most likely value is somewhere in the mid range. Both approaches also ignore the other lines of evidence that point to climate sensitivity likely being in the range of 2 to 4.5 degrees. E.g. a value as small as suggested by the NIPCC (0.3 degrees) is entirely inconsistent with the paleo-climate record of substantial climate changes in the earth’ history. And finally, both approaches implicitly assign high confidence to some of the most uncertain aspects of climate science, even though they routinely mock climate science as if nothing is known at all.

Whereas the existing uncertainty in the science is sometimes put forward as an excuse to continue business as usual, such an approach invariably suffers from viewing this uncertainty going in one direction only: making the problem seem smaller. What about the other direction? What if the risks actually increase faster (in the latter direction) than they decrease (in the former direction)? Combined with the basic picture of where we know we're heading, this paints a rather different picture of what can be considered "prudent" (even if that's ultimately a subjective judgment).

Of course it is not mandatory for all those who dismiss mainstream climate science to agree, but to see two important “spokespeople” for climate contrarians take such mutually inconsistent approaches is peculiar. Even more so when you realize that Lindzen signed the recent "Prudent Path" letter to US Congress, in which the NIPCC report was approvingly cited… Most people can’t have it both ways, but apparently climate contrarians can.

Comments

The IPCC has admitted that cloud formation and influence is poorly understood. The CERN CLOUD experiment will hopefully contribute some useful data.

Just standing outside on a sunny day, and feeling the difference when a cloud blocks the sun, indicates that cloud area and density must have a significance influence on climate.

It seems that the effects of aerosols is also very much still based on theory and models.

While it's fair to claim that a lot is understood about climate science, it seems to me that there are still far to many variables which aren't understood. Will positive or negative feedback dominate? Other than models, and can't find any data to conclusively show a direction.

I consider that any predictions for future climate must be treated with a high degree of uncertainty.

Rhjames,
You need to read some of James Hansens' papers. He also does not like to rely on models and theory so he used data from the past to estimate how the climate will change in the future. There is a lot of data on paleoclimate that shows future climate will be hard to handle. If you were more informed on the facts of Climate Science you might feel less uncertainty.

Well if aerosols partially warm but in fact provide a net cooling it will be reduced by black carbon IFR absorption and indirect warming of the Artic by aerosols in the absence of plentiful aerosols. The net warming effect is reduced by IFR scattered to space, partially through horizontal transfer to cloud formation, latent heat processes, forming thunderstorms from cirrus clouds and variable ice sizes. Roughly, estimated, taking UHI, thermometer siting issues, and calculating a median sensitivity with more parameters than Charny, I get a current 0.435 degree C warming, now maximum from all green house gases and only 0.375 maximum with all sulfates input. For a doubling of CO2 the Iris effect effect seems to increase in relative terms in accordance with the known physics, and the total warming is between
.85 and 1.06 degrees C which is not significant. This assumes the GCM's are somewhat correct as well as available data

Michael, Hansen has made numerous projection errors. He relies on partial data and GCM, high end estimates
Even Real Climate states so when praising his scenarios
Cloud formation is still highly uncertain. Knowing that,my calculated estimate will most likely need worlk

Having 3 scenarios of 3 different magnitudes and picking the closest value to published reports does not equate accuracy. It barely indicates precision. Lindzen is not infallible, but the Iris effect is well established. The projected high probability of a 3 degree at doubling is too high
I get it, there is a statistical clustering at around.3 degrees. In reality there is no empirical or physical reason to believe so.

Chemist1... "Having 3 scenarios of 3 different magnitudes and picking the closest value to published reports does not equate accuracy."

That would be interesting if it were the case. Problem is, what you're saying is wrong. The accuracy of Hansen's early work is said to be off due only to his estimate of climate sensitivity at 4.2C It's when you adjust the sensitivity figures in his work to 3C you get an almost uncanny match to reality. And, to back that up even further, that happens to be the number that's coming out of many other studies into climate sensitivity. About 3C.

In this we have multiple lines of evidence and research zeroing in on exactly the same figure.

I don't understand why you would throw your eggs into the Lindzen basket when his work is clearly an outlier. You would be as justified in suggesting that the extreme high sensitivity figures of 10C-12C are correct, which several studies do show.

If I were here claiming 10C for climate sensitivity what would you think of my argument? You'd think I was fruitcake! But here you guys repeatedly do exactly the same thing on the low side of climate sensitivity.

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Moderator Response: Also, scenario B was not picked as the most representative of the temperatures, but as the most representative of CO2 emissions, volcanoes, and so on--the conditions that were used to make the predictions.

"Having 3 scenarios of 3 different magnitudes and picking the closest value to published reports does not equate accuracy."

And that's not what was done, either. I suggest you actually read the link I provided.

"I get it, there is a statistical clustering at around.3 degrees. In reality there is no empirical or physical reason to believe so."

No, again, I suggest you actually read the link I provided. You keep making these factually incorrect statements and then ignoring the references provided which refute your errors. Please, take the time to understand what research has been done rather than simply making erroneous misinformed statements about it.

@ KR. Yep, as I recall the CERES satellites showed that, even if warming were to result in fewer tropical clouds, that the amount of energy getting *in* through those clouds would be greater than the amount of energy getting out. At least, that's how I understood it.

It’s not quite right to judge clouds’ cooling effects by how you feel as a cloud passes over your head. Clouds aren’t outside the climate system. The SW energy that did not reach your skin was partially absorbed by the cloud top and partially reflected to somewhere else, some of it outside the climate system. The warmer cloud warms the surrounding atmosphere. At the same time, some of the LW energy emitted by the Earth around you is intercepted by the water vapor held by that same cloud and again used to warm the surrounding atmosphere. A warmer atmosphere won’t let the Earth’s surface radiate as effectively so the surface will warm, though not as much as it cooled in the shady spot where you are standing. Some complicated measurements and accounting are needed to assess the overall effect and we aren’t there yet.

Aerosols get a big vote for variable concentrations and contributions to global weather-system effects. Mid-century suppressed warming has a decent correlation.

But this distraction on cloud cover has a big dead end sign on it - it shows little significant change, over decades and half-century measurements, that would indicate it will be a player in reshaping trends. As a forcing or feedback, it's insensitive.

“The global average trend of total cloud cover over land is small, -0.7% decade-1, offsetting the small positive trend that had been found for the ocean, and resulting in no significant trend for the land–ocean average.”

"Global mean total cloud cover over the ocean is observed to increase by 1.9% (sky cover) between 1952 and 1995. Global mean low cloud cover over the ocean is observed to increase by 3.6% between 1952 and 1995. … On the other hand, the fact that ships with a common observing practice travel over most of the global ocean suggests a possible observational artifact may be largely responsible for the upward trends observed at all latitudes."

"Of course they can't both be right, and probably they neither are".
They can't be both right. Indeed. Most skeptics have quite different views (Spencer, Scafetta, Abusamatow, Akasofu, Soon, Lindzen, many others and - most humble - myself). The funny thing is, that only one of them needs to be right to reject the mainstream view about climate sentitivity for greenhouse gasses and predictions about future warming.

Here's something interesting. The paper below refuted Lindzen's iris hypothesis. But the really interesting detail was that it was data refuting Lindzen's models.

It's actually no problem to use models to assist in the mathematical understanding of a complex problem, but Lindzen repeatedly claimed that everyone did wrong model sensitivity calculations, implying (wrongly) that he was the only one that used observations.

Excerpt: The modeled radiative fluxes of Lindzen et al. are replaced by the Clouds and the Earth's Radiant Energy System (CERES) directly observed broadband radiation fields. The observations show that the clouds have much higher albedos and moderately larger longwave fluxes than those assumed by Lindzen et al.

Again the contrarians' arguments are found to be inconsistent, at odds with observations, and relying on a whole lot of 'what ifs'. It is sad that people who want to wish AGW away will uncritically accept such seriously flawed science and logic.

This is a pretty damming refutation of Singer, Idso and Lindzen by Dr. Verheggen. More where that came form please.

Oh but I did look at those links. I see overestimation of the Earth's climate sensitivity and treating high unknowns and uncertainties as known and certain when they are not. Are you familiar guys and gals with the work in Darwin with the aircraft Proteus and the like? There are a bunch of papers I have looked at published in world renowned journals that discuss the effects of aerosols on precipitation, ice nucleation, and resulting cloud formation/ice size. Larger ice in cirrus clouds tend to absorb heat and smaller ones tend to reflect it back to space. Interesting reads. Some of the papers focus on the decrease, others the regional increase, in precipitation alone while others are more ambitious and look at both and why they might occur. Even when assuming climate change, AGW, specifically and the like, there are huge uncertainties discussed. Here are a few papers I have read and I think you should to below. Keep in mind these are not so called "skeptic"papers or papers that attempt to rule out global warming, but peer reviewed papers produced with the work of the top pilots, meteorologists and climate scientists in the world. They, themselves believe in global warming, but the amount of conclusions they state they cannot make, the level if uncertainty, the complexity of the system and variability analyzed is worth the read. They clearly discuss mechanisms and propose potential mechanisms of warming and cooling of aerosols, and discuss ice nucleation in a manner that one can easily extrapolate and infer the take home message: some GCM's are getting better, but there is a lot of work to do. These works are by authors who tend to believe the essence of the IPCC report but this work is of far higher quality than anything the IPCC has ever attempted. When non-skeptics point to such unknowns and level of uncertainty, the IPCC assigned probabilities dwindles. Nature laughs at mathematics and we need far more data. You really have to look at these papers and analyze them. Again they are not skeptics but their work provides evidence for a skeptical perspective, based upon real science.

Next post I will show why more scientists have become increasingly skeptical, even in climate science. Also even those who are not skeptical are realizing we cannot make accurate projections about global mean temperature based on the work above and other works I will post when I can open my file folder. I have specific papers in mind, but the above are a good start.

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Moderator Response: [Daniel Bailey] You are hardly presenting an evidenciary chain of logic here. Statements and phrases such as "world renowned journals", "top pilots, meteorologists and climate scientists", "more scientists have become increasingly skeptical, even in climate science" and "Also even those who are not skeptical are realizing we cannot make accurate projections about global mean temperature based on the work above" are the logical equivalent of unsupported hearsay and amount basically to appeals to authority. In a court of logic you have done the equivalent of testifying against yourself.

Might I humbly suggest that arguments about the fantastical "iris effect" be moved to another thread? This thread is primarily about aerosols, and how "skeptics" like Lindzen and Singer have also mangled the science of aerosols and aerosol forcing.

I know this is going off topic but I'd like to point out one statement by Chemist1.

"These works are by authors who tend to believe the essence of the IPCC report but this work is of far higher quality than anything the IPCC has ever attempted."

Can you please tell me what work the IPCC does? You do realize that the IPCC produces a report on climate change research. It does not actually do research, other than to research the published literature. The IPCC produces an "Assessment Report."

rhjames said: "The IPCC has admitted that cloud formation and influence is poorly understood."

As have I (starting with the very first sentence). But the funny thing is that e.g. in the case discussed in this post, it is the contrarians who implicitly build their argument on pseudo-certainty about a particular magnitude of aerosol forcing. I.e. they are guilty of what they routinely accuse mainstream science of.

See some preliminary results from CLOUD here. As you'll see, even the strongest link in the alleged chain of causality (ion induced nucleation) is hardly detectable, and even then in a few experiments only (I was there during one of the semi-succesful runs, and remember the comment "completely underwhelming" from someone who strongly believes in the cosmic ray-climate link).

rhjames said: "The IPCC has admitted that cloud formation and influence is poorly understood."

To me that statement seems to imply that the IPCC was hiding something and finally admitted to it. Would it not have been more accurate to say the IPCC "stated" or "commented" instead of "admitted"? Or am I just misinterpreting the statement?

Aerosols and ice nucleation influence the iris effect. Lindzen is the main progenitor of the iris effect. Lindzen also examines aerosol effects as negative feedbacks, therefore, my post is relevant for this thread.

Chemist1... But once again, all of Lindzen's work still is merely one data point suggesting low sensitivity. When we have many many other lines of evidence that point to higher sensitivity.

Lindzen focusses exclusively on this one aspect of climate while ignoring everything else. He absolutely could be correct but the problem then becomes how to explain the vast amount of empirical data that suggests higher climate sensitivity. What is the magical unknown mechanism?

Again, if I were here pounding the table about the research showing 10C for climate sensitivity, logically I would be on equal footing with the case you're presenting.

"In doing so, this study addresses some of the criticisms of the Lindzen et al. study by eliminating their more controversial method of relating bulk changes of cloud amount and SST across a large domain in the Tropics. The current analysis does not show any significant SST dependence of the ratio of cloud area to surface rainfall for deep convection in the tropical western and central Pacific."

"On the decadal time scale, the predicted tropical mean radiative flux anomalies are generally significantly different from those of the ERBS measurements, suggesting that the decadal ERBS nonscanner radiative energy budget measurements do not support the strong negative feedback of the Iris effect."

So the first two studies that came up using your linked search parameters do not support your contentions about Lindzen's semimythical "Iris Effect".

That is a statement of fact, the suggested link between GCRs and CCN is anything but an accepted fact. You are grasping at straws.

Also, as shown in the main post, Lindzen downplays the role of aerosols and neglects their importance, yet here we have you claiming that they influence the "iris effect". Yes, aerosols can affect the albedo and precipitation efficiency of clouds, but the purported "iris effect" does not require the impacts of aerosols, at least not in the context in which they are being discussed here. Where does Lindzen claim that aerosol forcing affects the "iris effect"? And if he did, that would run counter to his claim that aerosols are not a significant player-- in other words he would be contradicting himself.

This is yet another demonstration of the incoherence of arguments made by skeptics.

Albatross, I know. That is my issue with the IPCC. They only do meta analysis, and surveying of other's work, and at times do it well, but very often do it poorly. They also censor other's work that is not on line with a so called consensus.

No, Albatross, there are papers published by Lindzen that describe the power of aerosols, as well as, those by Christy and Spencer. Also I am adding data from other studies showing how aerosols and cirrus clouds interact to form a potent negative feedback.

Decadal time scales are not always the proper procedures when we consider how cloud response times are on the order of 24 hours or so and negative feedbacks are a year or so in length.

Rob: please see Spencer and Christy's work confirming an iris effect with more spatial resolution and temporal analysis.

#37 Chemist1: That is my issue with the IPCC. They only do meta analysis, and surveying of other's work, and at times do it well, but very often do it poorly. They also censor other's work that is not on line with a so called consensus.

That's their job, to review, analyze and report on what the greater scientific community is publishing in climate science. Their Job is not to do original research as a body. Why condemn them for something that they are not given the task to do? As for censoring others work please give a citation to back that assertion.

Chemist1 - I dont find suggestions for scholar searches remotely helpful. Please post proper link to specific papers that you think show that current understanding (as stated in IPCC assessment) is flawed. Pointing to well known science doesnt mean much to me. That's all acknowledged. Where is support for specific iris effect in wake of Lin, Rapp, Chambers, and other papers? With even Lindzen backing away, I am curious to know what you mean.

I am also very curious as to how you explain the current temperature trends if sensitivity is lower than 2.5 (not to mention the empirical determinations for climate sensitivity).

You say,"there are papers published by Lindzen that describe the power of aerosols"

Why then if they are so powerful as you claim and as you allege Lindzen claims, does Lindzen ignore them in his calculations as noted above?

I stated that it has been established in the literature that aerosols (depending on their size and type) can affect cloud properties by changing their albedo, as well as other properties-- so on that we agree.

I posted google scholar links
I read each article on the first page of each scholar link
Scheinder was more fair minded but he is gone now. He also failed to make his case about cloud cover as a positive feedback. In general, the IPCC report contains 18 significant flaws I can see that directly affects trend analysis and.the probability ranges, and level of confidence. NOAA data shows paleoclimate data where the global mean temps were higher and life flourished. The IPCC greatly neglects analysis of such data. Is the report 100% useless? NO. It does need considerable redesign.

#43 he has other papers that do not ignore such calculations. My own calculations do not ignore them. Lindzen just like all other scientists is not infallible but the data analyisis of Lindzen, Christy, Spencer, Pielke, and hundreds of others is very clear on potent aerosol effects and cirrus cloud responses to higher temps. No, they do not all agree on the exact degree location of mechansisms, but neither do: eric steig, Gavin or Tamino in all regards or contexts.

You are making many unsubstantiated statements. Pielke Snr's area of expertise is land use change, not aerosols. "...and hundreds of others"-- some supporting evidence please.

The fact remains that the "skeptics" are contradicting each other, some (like Lindzen) say net aerosol forcing is negligible, and you are saying that aerosols have a potent negative feedback. IIRC, Michaels in his recent testimony to congress suggested that aerosol forcing is a "fudge factor". Which is it?

Again, please provide links to papers papers published by Spencer, Christy, Pielke and Lindzen to support that claim please.

Chemist1: In general, the IPCC report contains 18 significant flaws I can see that directly affects trend analysis and.the probability ranges, and level of confidence. NOAA data shows paleoclimate data where the global mean temps were higher and life flourished. The IPCC greatly neglects analysis of such data. Is the report 100% useless?

Okay Chemist1, it's time to lay your cards on the table. List those 18 significant flaws and proceed to discuss each and every one as to why they are flawed with supporting evidence.

As for the NOAA comment please be specific about what you are commenting about. What data and temps.? A link please.

And please show how the IPCC neglects paleoclimate data. The IPCC AR4 WG1 has quite a bit of detail concerning paleoclimate data in Ch 6 which can be found HERE.

Rob, I have read each IPCC report to date. Have you? The 2007 report relies heavily on a misunderstood coefficient for buffering capacity. The tree rings are not a good data source, and neither are spleotherms. The hockey stick has been debunked and the 3 rd law of thermodyamics has been ignored due to ignorance of the first and second

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Moderator Response: [Daniel Bailey] With regard to your hockey stick comment, please refer the moderation guidance given in this comment. For the remainder, many posts exist here at Skeptical Science on the topics you discuss. Please use the search function to find one to better help you discuss those matters here. Thanks!